Polyurethane Belt - Joining, shortening, lengthen, bonding (how to) poly PU 

The temperature should be much lower. You should never see smoke or even bubbles. To get a good strong splice you need to push the belt against the plate to form a bead and then stop pushing and hold the belt still for about 15 to 20 seconds. This heats the belt deeper. Then when you join the belt push gently. If you push too hard you displace the hot material and you end up with cold rubber at the joint. This will cause a weak splice. Also this belt is only good for about 1/4 hp. Not the best choice for that lathe.

Yup, on burning the urathain. I'm going to attempt soon. I've worked PVC shaping, and the thing with PVC is patience or it turns black and only flexy in the small area you heated. I have an old band saw, I got some new wheels for but dont fit...to long, so I will be giving this a shot. It will only have to stretch going on and not constantly agitated like your issue. I'm glad you got it going, Im jealous ...your laith❤😂. Whish me luck!!

Way too hot. Around 350f and 20 seconds of heating to allow heat to wick and soften the ends. Too much heat breaks down the plastic

Great video on repairing a unique vintage tool!